To make low rank coals more competitive, several drying processes have been proposed to remove water from either lignite or subbituminous coal so that the specific heating value of the product is increased (U.S. Pat. Nos. 3,723,079; 3,985,516; 4,043,763; 4,192,650; 4,213,752; 4,396,394; and 4,401,436). These processes remove both surface moisture and moisture in the coal's pore structure. An inherent disadvantage of all these drying processes is that the coal must be heated in order to remove the water from the coal with a reasonable residence time in the dryer. Hence, after drying, the coal must be cooled to prevent spontaneous ignition. Several methods of accomplishing this have been proposed. Seitzer (U.S. Pat. No. 4,043,763) proposed simply blending wet and hot dried coal together after a drying process with no further processing. Other methods have included rehydration (U.S. Pat. Nos. 4,192,650 and 4,401,436). In particular, Bonnecaze (U.S. Pat. No. 4,401,436) added water to the dried coal before the coal was passed into a fluidized bed cooler where ambient air is drawn through the bed to cool the coal. Increased evaporation in the cooler then allows a cooler final product.
In general, the coal must be cooled to less than 100.degree. F. so that it will not spontaneously ignite after a drying process. This can be accomplished in a cooling fluid bed where ambient air is fed through the bed of coal. Cooling occurs by convective heat transfer and by evaporative cooling. Evaporation is the major contributor to cooling, and it is best to have the coal as wet as possible before the cooler. However, this contradicts the process in that coal is generally quite dry after the dryer, and the further removal of water is difficult. While this can be overcome by adding water to the coal prior to cooling, this offsets some of the effect of drying. That is, water is removed in one step, water is added in the second step, and the coal is cooled by evaporation in a third step.
It is also feasible to dry the coal by partial combustion. However, this may be undesirable from a product standpoint, among several reasons. This type of drying raises the coal to a very high temperature making it difficult to cool. It also partially consumes the coal during drying and hence lowers the dry basis heating value of the coal. For example, Blake (U.S. Pat. No. 4,324,544) teaches a process for drying coal in a fluidized bed by partial combustion, discharging from the bed and mixing with a stream of wet particulate coal, the combined stream then being cooled in a fluidized bed cooler where the fluidizing gas is the dryer's exhaust gas. Utilizing the dryer exhaust gas does not permit enough evaporative cooling due to the high humidity of the flue gas. In addition, gas condensation problems may arise when the cooler exhaust gas is passed through dust collection equipment.
Also, Seitzer (U.S. Pat. No. 4,213,752) teaches a process for removing moisture from low rank coal by feeding wet low rank coal into a moving bed of hot coal undergoing partial combustion, the moving bed being a fluidized bed.
Among other relevant art, Riess et al (U.S. Pat. No. 4,501,555) discloses a process for producing a dried particulate coal fuel from a particulate low rank coal using a fluidized bed apparatus. Nathan (U.S. Pat. No. 2,933,822) introduces finely divided solids wet with a liquid material into a dense phase fluidized bed containing similar solids having a lower liquid content. Ottoson (U.S. Pat. No. 4,495,710) provides a process for stabilizing particulate low rank coal in a fluidized bed.